1. Field of the Invention
This invention relates in general to document processing systems; and more particularly to document jam detectors in such systems.
2. Description of the Prior Art
Document processing machines often move documents at high speeds with small interdocument spacings for efficiency. For various reasons, such as, for example, a bent corner, documents being processed can stop (jam) in the document transport path. It is desirable to detect such jams very rapidly because additional documents will be colliding with the jam. Such additional documents will probably be damaged, will increase the time required to clear the jam from the machine, and can also potentially damage the machine itself. Furthermore, the faster the document velocity, the more documents are likely to collide with the jam before the machine can be shut down. Thus, it is desirable to detect document jams very rapidly in document processing machines.
U.S. Pat. No. 3,815,895 issued to Dufour discloses a book manufacturing machine having a jam detector. The machine has several synchronized processing stations that sequentially operate to cut and fold signatures (paper) for books. The sequential processing stations are synchronized by a common drive mechanism; and light sources and photocells are used to detect the presence or absence of signatures at predetermined locations at discrete monitor times. The discrete monitor times are indicated by the position of the common drive mechanism. The disclosed jam detector, however, detects only the presence or absence of an object at discrete times and hence normally has a rather lengthy inherent delay before it can indicate that a jam has occurred.
U.S. Pat. No. 3,150,873 issued to MacGregor discloses a document processing apparatus having a jam detector associated with a document transport path. The jam detector utilizes at least two sets of light source-photocell pairs. Each light source-photocell pair is located along the transport path and detects the presence or absence of a document in the transport path at that location. A jam is detected if a document passes between a first light source-photocell pair and does not pass between a second downstream light source-photocell pair within a given time. The given time is defined by the distance between the first and second light source-photocell pairs and the expected velocity of the document. This jam detector, however, detects only the presence or absence of a document and also normally has a rather lengthy inherent delay before it can indicate that a jam has occurred.